Allergic reactions to alcohol most often occur as an allergy to ingredients found in certain types of alcohol or to substances that alcohol is mixed with in cocktails. An inability to metabolize alcohol can also cause flushing that resembles a rash, particularly in some ethnic groups. Because allergic reactions often worsen with increased exposure, see your medical practitioner for allergy testing if you have a rash after drinking alcohol.
Alcohol intolerance can cause facial flushing and nasal congestion that resembles an allergic reaction rash. Most common in Asians, alcohol intolerance occurs because a person lacks an enzyme, aldehyde dehydrongenase, which breaks down the toxins in alcohol. Rapid, pounding heartbeat and nausea and vomiting can also occur. This type of alcohol intolerance is most often genetic. Histamines, which form from fermentation or brewing processes, can also cause a reaction. Histamines are the same substances released when your body encounters an allergen.
People who have allergies to grains used to make alcohol, such as grapes, hops, wheat, barley or rye, or who have allergies to sulfites used as preservatives in wines can have allergic reactions if they drink alcohol. Mixes that contain tonic water or juice can also cause an allergic reaction with a rash. An article published in the June 2007 "Journal of Dermatology" by researchers from the University of Osaka described rashes that occurred in two people after drinking alcohol mixed with tonic water, which contains quinine.
In addition to a rash, people with an allergy to alcohol or some substances found in alcoholic drinks may experience severe reactions, with shortness of breath, throat tightening, facial swelling around the eyes and mouth, hives, low blood pressure and collapse. Severe reactions, called anaphylaxis, can be life-threatening; seek medical treatment immediately.
Whether you develop an allergic rash or have facial flushing due to alcohol intolerance, avoiding alcohol or the substance that you're allergic to is imperative. If you're allergic to a particular type of alcohol, you may be able to drink other types, but if you have an inherited intolerance, even a small amount of any type of alcohol can cause a reaction. Antihistamines can help decrease symptoms in a mild allergic reaction, but severe reactions require adrenaline in the form of an auto-injector that you can carry with you. Your medical practitioner must prescribe this. This should not be used as a backup plan for drinking alcohol; not drinking will prevent any type of alcohol-related reaction.